News - Solder Paste, Solder Flux

September 12 2018

How Does Printed Solder Paste Volume Affect Solder Joint Reliability

Stencil design files are typically created along with the circuit board designs. The stencil aperture sizes are often the same size as the copper pads (1:1). If solder paste was printed using these aperture sizes, then issues could occur such as solder balling or bridging. It is common practice to reduce the stencil aperture size by 10% to 50% in order to prevent such defects. This significantly reduces the volume of printed solder paste.

When fine pitch micro ball grid array (µBGA) or 0201 Imperial (0603 metric) and smaller passive components are used then the stencil thickness may be reduced in addition to reducing the aperture size. This is done to maintain the aperture area ratio above the industry standard minimum value of 0.66.

Reducing stencil thickness also reduces the volume of the printed solder paste. The solder joints created from the reduced solder paste volume must meet IPC-A-610 and J-STD-001 criteria, but are the solder joints reliable? What is the lower limit of solder paste volume required to produce a reliable solder joint? Will the solder joints survive for the lifetime of the assembly?

In order to help answer these questions, reliability testing must be performed with a range of solder volumes to determine the lower limit of the printed solder paste volume which can be used.

A study was conducted to answer the question of how printed solder paste volume affects solder joint reliability. The circuit board chosen for this work includes a variety of component sizes and types (Figure 1).

The components tested are as follows: 0402, 0603, 0805, and 1206 Imperial chip components (1005, 1608, 2012, 3216 metric); PLCC; SOT, and SOIC lead-frame components.

Figure 1: PCB008 and Components Used.

In an effort to determine the lower limit of acceptable solder paste volume, printed volumes were varied between 25 and 125% of nominal (Table 1).

Table 1: Stencil Volume Levels Created for this Work.

Solder joint quality was assessed using IPC-A-610 standard methods and cross-sectional analysis (Figure 2). Solder joint strength was measured using shear and pull tests. Thermal cycling between -40 °C and 125 °C was done for 1000 cycles and solder joint quality and strength was measured again.

Figure 2: Solder Joint Pictures for Each Component by Stencil Volume

In summary, the solder joint reliability data was correlated to printed solder paste volume. This was done in an effort to establish basic guidelines for the printed solder paste volumes required to generate reliable solder joints. This work will be will be presented at a technical conference. Please contact us if you would like more information.

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August 14 2018

How Does Surface Finish Affect Solder Paste Performance?

Solder Paste, Surface Finish 0

There are many solderable surface finishes used on printed circuit boards. These surface finishes are necessary to protect the copper pads from oxidation and corrosion. Copper oxidizes very quickly and the oxide thickness grows over time. Copper oxides are very difficult to solder to especially with low activity no-clean fluxes. Solderable surface finishes protect the copper pads and holes while enabling good solderability.

Historically, the most common surface finish was tin-lead (Sn63/Pb37) hot air solder level (HASL). The process used to coat circuit boards with HASL is as follows. The copper is etched to remove oxides, then the circuit board is coated in hot air flux. The circuit board is immersed in molten solder. As the circuit board is removed from the solder hot air is used to “level” the solder making it relatively flat and clearing the holes.

Finally, the circuit boards are washed to remove flux residues. Tin-lead HASL has the inherent issue of being non-planar or “bumpy”. This can have detrimental effects on printing of solder paste and component placement.

Tin-lead HASL is still used today as it is also known to solder very well and is tolerant of multiple soldering cycles.

Lead-free HASL is a common finish that has replaced tin-lead HASL in many applications. The process used to coat circuit boards with lead-free HASL is essentially the same as the process for tin-lead HASL. The only exception is the solder alloy that is used. Tin-lead HASL uses Sn63/Pb37 alloy, while lead-free HASL uses tin/copper based alloys like SN100C. Lead-free HASL also has the inherent issue of non-planarity, but solders quite well through multiple soldering cycles. HASL finishes are very durable and hold up well to mechanical damage.

Organic solderability preservative (OSP) is a popular finish due to it’s low cost relative to the other surface finishes. The process used to coat circuit boards with OSP is as follows. The copper is cleaned and etched to remove contaminates and oxides. Then the copper is coated with an azole mixture. The excess liquid is removed and the coating is dried.

OSP must be handled carefully because it is susceptible to mechanical damage. OSP coated circuit boards must be protected from the air and moisture to prolong shelf life. OSP is flat so it does not have the same issues with non-planarity as HASL.

OSP is known to show issues with multiple soldering cycles and is mainly used for cost-sensitive applications that require a limited number of soldering cycles.

Electroless nickel immersion gold (ENIG) is a very popular surface finish. ENIG’s counterpart electroless nickel electroless palladium immersion gold (ENEPIG) is becoming more popular. The process used to coat circuit boards with ENIG is as follows. The copper is cleaned and etched to remove contaminates and oxides. Then the copper is coated with a metallic catalyst, like palladium. Nickel is plated through an electroless plating process which incorporates phosphorous into the nickel deposit. A thin layer of gold is plated onto the nickel deposit through an immersion process where nickel metal is dissolved and replaced by gold metal. ENIG is susceptible to the issue of hyper-corrosion of the nickel which is known commonly as “black pad”.

ENIG processes have been improved over the years to minimize the risk of “black pad”.

ENIG is one of the more costly surface finishes. ENIG is a flat finish which promotes good solder paste printing. ENIG is also solderable through multiple cycles and has a long shelf life.

Immersion tin is a popular surface finish used mainly in Europe and Asia. The process used to coat circuit boards with immersion tin is as follows. The copper is cleaned and etched to remove contaminates and oxides. The copper surface is prepared for tin plating through a pre-treatment step, then plated with immersion tin. Copper is dissolved into the plating solution as tin metal is plated onto the circuit board pads.

The immersion plating process is self-limiting which limits the maximum thickness of tin. Immersion tin is flat and solders well initially. Lead-free soldering temperatures can damage the finish causing subsequent soldering steps to be challenging. Immersion tin is also susceptible to handling damage because it is relatively thin.

Immersion silver is a finish similar to immersion tin but silver metal is plated over the copper pads. The process used to plate immersion silver onto circuit boards is as follows. The copper is cleaned and etched to remove contaminates and oxides. The copper surface is prepared for silver plating through a pre-treatment step, then plated with immersion silver. Copper is dissolved into the plating solution as silver metal is plated onto the circuit board pads. The immersion plating process is self-limiting which limits the maximum thickness of silver. Anti-tarnish agents are either incorporated into the silver plating step or applied as a final step.

Immersion silver is flat and solders well initially, but can be tarnished through air exposure and by the heat applied for soldering.

Tarnished immersion silver is very difficult to solder. Immersion silver is susceptible to handling damage because it is relatively thin, and must be protected from air and sulfur exposure. Both immersion silver and immersion tin are cost-effective finishes.

With so many surface finishes available on the market, how can performance be optimized for each surface finish? Recently work has been done to test the performance of a variety of solder pastes with each of the surface finishes shown below (Figure 1).

Figure 1: Surface Finishes Tested with a Variety of Solder Pastes

Four different solder pastes were used in this testing.

It includes water-soluble Sn63/Pb37 and SAC305 solder pastes, and no clean Sn63/Pb37 and SAC305 solder pastes. FCT Assembly’s PR test board was used for this evaluation (Figure 2).

Figure 2: PR Test Circuit Board Used for Surface Finish & Solder Paste Evaluation

Performance was measured in several categories.

It includes: printing of solder paste, wetting, random solder balling, graping, and voiding. The performance metrics below (Figure 3) were used to score each surface finish & solder paste combination.

Figure 3: Print, Reflow, and Voiding Metrics Used for Scoring Each Surface Finish & Solder Paste Combination

Performance was summarized and recommendations made to help optimize the performance of solder paste with each surface finish. This work will be will be presented at a technical conference.

Interested in learning more about solder paste performance?

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February 14 2018

Surface Mount Soldering

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Surface mount soldering is the process of creating circuits using surface mount technology (SMT) components that are mounted directly onto the surface of printed circuit boards (PCBs).

SMT technology is a key driver in the ever-important work of miniaturization of circuits, allowing for smaller, lighter devices with ever-increasing capabilities.

Today’s cell phones have exponentially more computing power than the massive supercomputers that controlled the Apollo space missions. The manufacture of PCB circuits via surface mount soldering was a key breakthrough that makes this possible.

Process

A pre-printed PCB outlining the circuit, with solder pads indicating component locations and connections, is matched with a stencil that directs the application of solder paste.

Solder paste is a gelatinous mixture of powdered conductive metal (commonly tin) and flux, which is a liquid or semi-liquid chemical compound that acts as a binding, cleaning, and wetting agent.

The solder paste is commonly spread in a wide swatch over the stencil, which directs it to only those solder pads where components are to be attached and allows for easy reclamation of excess paste for re-use.

Once the paste is applied, a conveyor belt runs the PCB through a pick-and-place machine where components are simply set onto the board. Tolerances for placement are generally relatively more forgiving than other solder methods, as the coming reflow process will automatically align the components on their pads.

When both paste and components are in place, the PCB then transits through a reflow oven, which heats the board at a rate designed to melt the solder paste without damaging the components. The reflow process liquefies the metal component of the solder paste, cementing the contact points of the electrical components to the PCB and ensuring electrical conductivity, while common evaporating the excess flux. The surface tension of the molten solder helps keep the components in place.

Reflow ovens for surface mount soldering typically come in one of three categories: convection, infrared, or vapor phase

Convection

Convection is the most common as it allows for equal heating of the entire board, which prevents the issue of some solder paste locations not reaching the melting point. This equal heating, though, requires tighter overall control as PCB components are subject to the same heat and can be damaged by it.

Infrared

Infrared allows for much more pinpointed heating to be applied, significantly reducing the risk to components, but drastically reduces tolerances for misalignment and resultant failure of the solder paste to melt and form the necessary bond.

Vapor

Vapor phase removes both risks, but requires expensive, specialized equipment to handle the fluorocarbon liquids with high boiling points and mitigate the environmental risks associated with those compounds.

Following reflow, a washing of the completed board is recommended. This both removes any stray solder balls that can cause short circuits, as well as remove flux residues that may have spread onto the PCB during the heating process.

Depending on the type of solder paste used, different solvents are required:

No-clean fluxes are most common, and they do not require strict washing. The flux residue is considered harmless, and can simply be left on the board. However, a manufacturing process that skips the washing step requires a very clean assembly environment and may need an inert reflow environment to prevent conductive particles from landing on the board. If a conformal coating is to be applied, washing is still recommended, as the flux residue could cause those coatings to adhere less effectively.
Water-soluble fluxes can be removed with a directed water spray, typically including a detergent compound
Rosin-based fluxes require specific chemical solvents, generally involving fluorocarbons

Advantages

Because surface mount soldering allows for machine-guided placement of components and printed stencil-guided application of solder, a much more tightly packed circuit board is possible. The advantages here are numerous, including:

Smaller components (from one-quarter to one-tenth the size and weight)
Higher component density
More connections per component
Lower radiated emissions (due to the smaller radiation loop area and inductance inherent to smaller components)
Lower resistance and inductance at the connection
Cheaper components (again, generally due to smaller components – from one-half to one-quarter the cost of equivalent through-hole parts)
Final products that both take up less space and have less mass

Surface mount soldering has also proven to be the most efficient and cost-effective process for mass-production of printed circuit boards. Because the setup is almost entirely automated equipment, there is less time, setup and labor cost involved, as well as simpler and faster operation once the process is in place. On the high-end, some pick-and-place machines are capable of placing more than 2,000 components per minute.

As mentioned previously, the reflow step key to the surface mount soldering process allows small errors in component placement to be corrected automatically as the surface tension of molten solder pulls components into alignment with solder pads.

Disadvantages

Because surface mount soldering relies on the surface tension of the melted solder paste to hold components to the board, it is not recommended for large components, nor those with high-voltage capacities. In PCBs that use transformers, heat-sinked power semiconductors, large capacitors, or fuses, either additional adhesive and/or a combination of both surface mount and through-hole soldering is used.

Through-hole soldering also adds a degree of mechanical connection between components and boards by extending leads through PCBs, contrasted to the simple seated SMT model. This means surface mount soldering creates a mechanically weaker connection and is not preferred for components that will be undergoing sheer force.

The key advantage of surface mount soldering, with the automation process allowing smaller and more dense packing of components, also means that repairs can be difficult.

Surface mount soldering is preferred over through-hole soldering because of the level of precision necessary to achieve that density, and that same density requires an extremely high level of precision, expertise, and often expensive equipment for repairs.

The higher speed and miniaturization in the production process can increase the risk of defects, so a failure detection process is recommended for any SMT manufacturing process. Surface mount soldering is prone to a variety of potential issues and these issues can begin with solder paste printing.

FCT Assembly and our partner BlueRing Stencils offer a variety of high-quality products which can improve the solder paste printing process.

Our solder pastes have excellent printability and long stencil life. Our stencils and nano-coatings are designed to improve the print process.

FCT Assembly offers technical support for the entire surface mount soldering process. Please contact us if you need help with surface mount soldering.

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February 10 2018

Solder Paste Flux

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Solder paste flux is a gelatinous chemical compound used mainly in combination with solder powder to create solder paste, generally mixed as a 50/50 ratio. Once combined, the paste is a gray, putty-like material.

Solder paste flux serves a threefold purpose:

Removing any oxidized metal from the surfaces to be soldered
Sealing out air, preventing further oxidation
Improving wetting characteristics of liquid solder by facilitating amalgamation.

Chemical Properties

Solder paste flux has a very complex chemical composition. It can be comprised of:

Solvents – liquid used an inert medium to combine the active ingredients, such as water
Rosins/Resins – high-temperature tolerant, non-volatile chemicals, most commonly forms of glycol
Surfactants – a detergent used to lower the contact angle of the flux with the surface its applied to, allowing greater coverage
Activators – chemicals disrupting/dissolving metal oxides, typically acids of various corrosivity
Rheology Modifiers – any of a variety of compounds that allow a liquid to display Non-Newtonian properties; that is, to change viscosity as pressure is varied

Flux is nearly inert at room temperature but becomes strongly reducing when its temperature is increased. This prevents the formation of metal oxides on both the base and filler materials to which it is applied. It also prevents the beading of the applied solder, facilitating easy flows and application.

Solder paste flux is different than liquid flux in both physical form and activity level. Solder paste flux typically contains 60-80% by weight active ingredients, compared with liquid flux’s 2-25%.

By weight, a mixed solder paste commonly consists of 90% metal. However, the non-metallic ingredients of the flux, due to their lower density, result in their comprising of nearly half the paste volume.

Standards and Definitions

A number of standards exist to define the various flux types. The principal standard is J-STD-004, which categorize fluxes by composition, activity (strength), the presence or lack of halide activators, and reliability from electromigration standpoint and surface insulation resistance (SIR). The J-STD-004 standard for electromigration and surface insulation resistance is >100 MΩ after one week at elevated temperature and humidity with a DC bias applied.

There are three types of solder paste flux:

Water-Soluble

Fluxes are composed of glycol bases and organic (carbon-based) materials.

Rosin Based

Fluxes are derived from a pine tree extract.

No-Clean

Fluxes are made with resins and various levels of solid residues.

Use

While the main use of solder paste flux is in the creation of solder paste, it is also sold directly as a gel or tacky flux.

Solder paste flux can also act as a temporary adhesive. Its tacky nature holds components together until the heat of the soldering process melts the solder and the parts are fused together.

Once mixed, solder paste is most commonly used in a stencil-printing process. Paste is spread over a stencil in the pattern required on the final end product, a printed circuit board.

The solder paste flux component must have enough tackiness to secure the components while the fixture is processed through the manufacturing line.

Once printed, the circuit board forming process is followed by pre-heating and reflow (melting).

For the flux, the temperature and rate of change in temperature are key in the reflow process. A slow rise in temperature is necessary to prevent solder balling, but must be great enough to activate the flux, then melt the solder itself, and recool quickly enough to retain the desired shape.

Application

Solder paste flux gel can be brushed on where needed and, unlike liquid flux, will not flow away from the application area.

This means that, when used directly as a soldering aid, a small amount of solder paste flux can be used in place of a relatively large amount of liquid flux. Common uses of flux gel include BGA re-balling and the rework/repair of solder joints.

In mixing solder paste flux and solder powder for application to circuit assemblies, there are important considerations:

Working Life

Different types of flux, particularly water-based, can gradually evaporate after mixing, eventually hardening to the point of unusability. Pre-mixed solder pastes have this measurement in time provided by the manufacturer. The working life of self-mixed pastes can be approximated, but should be underestimated for safety.

Viscosity

As a liquid, solder paste is not entirely immune to flow. The thickness of the flux/powder mixture helps determine how well it resists this tendency to hold its shape through the curing process. However, sometimes a lower viscosity is preferred.

When using a squeegee to apply solder paste onto a stencil, the force used to apply can break down the viscosity, which thins the paste and allows it to more easily flow through the stencil apertures. Again, pre-mixed pastes have manufacturer-provided measurements, while custom mixes often require self-testing.

Both working life and viscosity can be expressed through the thixotropic index, a measurement of a mixed solder paste’s viscosity at rest versus applied.

Depending on the ratio and composition of the solder paste flux and solder powder, stirring may be needed at frequent intervals to ensure proper viscosity and test for working life.

The amount of paste is also a key consideration. Too little may cause a joint to lack strength or an electrical connection to have poor conductivity. Too much can result in accidental short-circuits on the board.

Storage & Cleanup

Solder paste flux needs proper storage to maintain usability. An airtight container must be used to prevent evaporation. Storage in an area of a low temperature reduces the rate of oxidation, and thus the rate of degradation of the flux – but take care not to introduce it to freezing temperatures, which can cause separation of the chemicals.

As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards exist to measure the safety of the residues left behind.

Each type of flux requires its own solvents for cleanup of excess:

Water-Soluble

Have the widest available range of cleaning agents, generally a mix of deionized water and detergents

Rosin Based

Require specific chemical solvents, typically involving fluorocarbons.

No-Clean

Fluxes, true to their name, generally require little, if any, cleaning.

The residual flux is considered harmless, and can simply be left to dry on the board. Their use necessitates a very clean assembly environment and may need an inert reflow environment.

Washing is still recommended, however, as residual no-clean flux can cause conformal coatings to adhere less effectively.

FCT Assembly manufactures and sells a wide variety of soldering products. These include solder pastes, wire solder, bar solder, liquid flux, and gel flux.

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January 10 2018

Solder Paste: Type 3 vs. Type 4 vs. Type 5

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Solder paste can be made with a variety of solder powder sizes. Solder powder sizes are classified by Type in the IPC standard J-STD-005 Requirements for Soldering Pastes. Table 3-2 details the solder powder size ranges for each type, and an excerpt from the table is shown below*.

*IPC J-STD-005A Requirements for Soldering Pastes, Feb 2012, Table 3-2

The main particle size range is normally associated with the type. For example, Type 3 solder powder mainly falls within the 25 – 45 µm size range, therefore Type 3 solder paste may be labeled as “Type 3 (25-45 µm).” Below are images of these three sizes of solder powder.

Why use Type 4 or 5 solder powder rather than Type 3?

The main reason to use Type 4 or Type 5 solder paste is to improve printability for miniature components. Type 4 and Type 5 solder pastes can be printed through smaller stencil apertures than Type 3 solder paste. Generally speaking, Type 3 solder paste can be used for components ranging in size down to the 0402 imperial package size. Most solder paste users prefer Type 4 solder paste for 0201 imperial, 01005 imperial, micro-BGAs and similar components. Type 5 solder paste is typically used for even smaller soldering applications, or when Type 4 solder paste does not print adequately.

A comparison of print data for Type 3, 4, and 5 no-clean, lead-free solder pastes is shown in chart below. Transfer efficiency was measured across a variety of stencil aperture area ratios ranging from 0.38 to 0.80.

Let's Talk Solder Paste

In general, Type 5 gives higher transfer efficiency than Type 4 solder paste, and Type 4 gives higher transfer efficiency than Type 3 solder paste. At smaller area ratios this difference in transfer efficiency becomes less distinct.

Type 4 or 5 solder pastes enhance printing for miniaturized components. This is especially true when these solder pastes are coupled with stencil technologies designed to enhance printing. Fine grain laser cut steel stencils along with NanoSlic Gold coating give a substantial increase in transfer efficiency as compared to standard steel stencils. The increase in transfer efficiency is dramatic when Type 4 or 5 solder pastes are used along with fine grain steel stencils coated with NanoSlic Gold. This combination allows for printing of even the finest of components.

Smaller solder powder sizes like Type 4 and Type 5 give improved printability but also can create some issues due to chemical reactions with the solder powder. The surface area of the smaller solder powder types is significantly higher than the larger solder powder types. For the same mass of solder powder, the surface area of Type 4 solder powder is roughly 20% higher than Type 3, and the surface area for Type 5 solder powder is roughly 75% higher than Type 3.

The rate of chemical reaction with the solder powder is directly proportional to the surface area of the solder powder. Therefore, smaller solder powders react more quickly than larger solder powders. This higher rate of reaction leads to potential issues when using Type 4 and 5 solder pastes.

The shelf life of Type 4 and 5 solder paste is typically shorter than Type 3 due to a higher rate of reaction between the flux and solder powder. Type 4 and 5 solder pastes can generate more random solder balling and graping than Type 3 with certain solder paste formulations. This is due to a higher potential for oxidation of Type 4 and 5 solder powders. Most solder paste manufacturers correct these issues by adjusting the formulas for Type 4 and 5 solder pastes. Newer Type 4 solder pastes typically perform equal to or better than Type 3 solder pastes.

FCT Assembly is ready to meet your needs for Type 3, 4 and 5 solder pastes. We manufacture and sell a wide variety of soldering products including wire solder, bar solder, liquid flux, gel flux, and solder pastes.

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